1. Field of Invention
The present invention relates to a light emitting device power supply circuit, a light emitting device driver circuit and a control method thereof. Particularly, it relates to such light emitting device power supply circuit, light emitting device driver circuit and control method thereof which generate a latching current for firing a TRIAC device, wherein latching current is guided to an output node, such that the power utilization efficiency is improved and less flicker occurs.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) power supply circuit. As shown in FIG. 1A, the LED power supply circuit includes a tri-electrode AC switch (TRIAC) dimming circuit 12, a rectifier circuit 14, and an LED driver circuit 16. The TRIAC dimming circuit 12 receives an AC signal from an AC input line node VL. When the AC signal exceeds a predetermined trigger phase, the TRIAC dimming circuit 12 fires and turns ON. FIG. 1B shows a schematic diagram of the input and output signal waveforms of the TRIAC dimming circuit 12. The AC signal at the AC input line node VL is shown by a dash line, and the AC dimming signal at the node VL′ after the TRIAC dimming circuit 12 is shown by a solid line. The rectifier circuit 14 receives the AC dimming signal at the node VL′, and rectifies it to generate a rectified dimming signal which is inputted to the LED driver circuit 16 for driving the LED circuit 11 and adjusting its brightness.
One of the drawbacks of the aforementioned prior art is that the TRIAC dimming circuit 12 includes a TRIAC device, and the TRIAC device requires a large latching current to fire. If what the power supply drives is a high power consuming load circuit, such as a conventional incandescent lamp, the latching current for the TRIAC device is sufficient. However if what the power supply drives is a low power consuming load circuit, such as the LED circuit 11, the latching current for the TRIAC device is insufficient because of the low current of the LED circuit 11. If the power supply circuit does not generate a sufficient latching current to fire the TRIAC device, a so-called “misfire” occurs and the LED circuit 11 will flicker perceptibly. FIG. 1C shows the waveforms of the signals at the nodes VL and VL′ when the misfire condition occurs.
FIGS. 2A and 2B show schematic diagrams of another prior art LED power supply circuit which solves the misfire problem of the aforementioned prior art. Different from the prior art LED power supply circuit shown in FIG. 1A, the prior art LED power supply circuit shown in FIG. 2A further includes a bleeder circuit 18 between the rectifier circuit 14 and the LED driver circuit 16; the bleeder circuit 18 is designed for generating the latching current in every cycle for firing the TRIAC device of the TRIAC dimming circuit 12. The latching current generated by the bleeder circuit 18 is consumed through a path connecting to ground. FIG. 2B shows a more specific example of the bleeder circuit 18.
More specifically, in the bleeder circuit 18, resistors R1 and R2 are connected in series between two output nodes of the rectifier circuit 14, and a division voltage at the node between the resistors R1 and R2 turns ON a switch Q1 to generate the latching current. A resistor R3 and Zener diodes ZD1 and ZD2 are connected in series between the two output nodes of the rectifier circuit 14, and a division voltage at the node between the resistor R3 and the Zener diode ZD1 turns ON a switch Q2 to generate the holding current for maintaining the TRIAC operation; the holding current flows through a resistor R4.
Although the prior art shown in FIGS. 2A and 2B solves the misfire problem and hence solves the flicker problem of the LED circuit, the power consumed by the bleeder circuit 18 is wasted.
In view of the foregoing, the present invention provides a light emitting device power supply circuit, alight emitting device driver circuit and a control method thereof to improve the drawback of the prior art. Particularly, the present invention generates a latching current for firing a TRIAC device, and the latching current is guided to an output node which supplies power to the light emitting device, so that unnecessary power consumption is reduced while the flicker problem is solved.